1. Field of the Invention:
Macrocyclic esters are obtained by the thermal depolymerization of the corresponding linear polyesters accompanied by ring closure. For such processes, the polyester is heated at an elevated temperature in the presence of a catalyst and the macrocyclic compound formed during the course of the depolymerization is removed from the reaction zone.
2. Description of the Prior Art:
Chlorides, nitrates, carbonates and oxides of magnesium, manganese, iron, cobalt and tin (all in the divalent state) are employed for the depolymerization of linear polyesters in the process of U.S. Pat. No. 2,092,031. In the process of U.S. Pat. No. 4,165,321 Lewis metal salts such as the oxides, hydroxides, halides or carboxylates of Group IIIa, IVa, IVb, Va, VIIb and VIII metals are disclosed to be useful catalysts. Yasakawa et. al. reported the use of lead catalysts (oxide, hydroxide, carbonate, nitrate, borate or organic acid salts) for the preparation of large ring lactones via thermal depolymerization in Chemical Abstracts, Vol. 78 (1973), 158966q and 158968s. Cyclic esters are also obtained via thermal degradation of polyesters using SnCl.sub.2.2H.sub.2 O in Chemical Abstracts, Vol. 86 (1977), 156163s. In U.S. Pat. Nos. 4,105,672, 4,136,098, and 4,157,330 tin carboxylates and organotin compounds are employed in conjunction with dialkyl-(3,5-di-t-butyl-4-hydroxybenzyl) phosphates to catalyze the reaction. In British patent No. 1,108,720 the formation of cyclic ester anhydrides of alpha-hydroxycarboxylic acids in vacuo by depolymerizing the corresponding linear polymer at 200.degree. -240.degree. C. in the presence of lead (II) stearate is described.
In view of the problems associated with the use of heavy metal catalysts, aluminum oxide has been used to catalyze depolymerizations carried out at atmospheric pressure using superheated steam in Czech patent No. 108,762. The use of metallic aluminum was also reported for the thermal depolymerization of polyesters to form cyclic esters and lactones in Japanese patent No. 36-1375 (1961). Aluminum alcoholates are disclosed for the preparation of large-ring lactones in Japanese patent publication No. 72 25,071.
Mixed-metal catalysts having a carbonic acid radical and based on aluminum and sodium, wherein aluminum is the predominant metal, are also disclosed for the depolymerization of polyesters to produce macrocyclic compounds in Japanese patent disclosure Nos. 1979-103,884 (appln. No. 1978-8,809); 1979-115,390 (appln. No. 1978-22,023); and 1980-120,581 (appln. No. 1979-26,741). The mixed-metal catalysts of these Japanese references all have aluminum:sodium weight ratios greater than about 3.5:1. The catalysts are typically prepared by treating an aqueous mixture of aluminum hydroxide and caustic soda with carbon dioxide. Mixed-metal catalysts comprised of aluminum alkoxides or aluminum carboxylates with an alkali metal or magnesium alkoxide or carboxylate are utilized in the process of U.S. Pat. Nos. 4,393,223 and 4,499,288. Dawsonite, a naturally occurring basic carbonate of sodium and aluminum, is also an effective catalyst for the thermal depolymerization (see U.S. Pat. No. 4,594,434).
A problem common to all of the above-described catalytic thermal depolymerization processes is the viscosity of the reaction mass. As the depolymerization proceeds, the viscosity of the reaction mixture increases due to chain-growth reactions occurring between partially depolymerized fragments and an intractable plastic mass is formed. Stirring becomes extremely difficult and, in some cases, impossible. Mixing is non-existent or, at the very best, highly inefficient. Thus, heat transfer within the highly viscous reaction mass is very poor and localized "hot spots" occur resulting in charring of the reaction mixture and reactor fouling. As a result of these viscosity/heat transfer problems, reaction times are extended, yields are reduced, and undesirable malodorous decomposition products are obtained.
In an effort to overcome these problems, processes have been developed whereby specialized mixing equipment is employed and/or the reaction is conducted in the presence of other compounds. U.S. Pat. Nos. 4,165,321 and 4,218,379, for example, describe processes wherein the reaction is carried out in the presence of a monocarboxylate moiety derived from an aliphatic or aromatic monocarboxylate and wherein agitation is employed which provides top-to-bottom mixing throughout essentially the total volume of the reaction mass in an inverted multiple-blade conical vessel wherein the blades have a helical configuration and are arranged to rotate throughout essentially the entire reaction mass and in close proximity to the interior surface of said conical vessel and in a direction which provides a downward flow within the reaction mixture.
In Japanese patent disclosure No. 1980-120,581 (appln. No. 1979-26,741) the depolymerization and ring formation are carried out in the presence of at least one compound selected from the group consisting of polyoxyalkylene glycols, polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, monobasic acids, monobasic acid esters, monobasic anhydrides, monovalent alcohols and monovalent alcohol esters. A process is disclosed in Japanese patent No. 73-1972 wherein the depolymerization is carried out in the presence of a paraffin oil. Whereas such processes overcome some of the viscosity/heat transfer problems associated with the reaction, undesirable decomposition products which have objectionable odors result. Such processes are therefore not suitable for the production of macrocyclic compounds intended for critical fragrance applications.
It would be highly desirable therefore if a process were available whereby the viscosity/heat transfer problems associated with these depolymerization reactions were avoided and the formation of undesirable malodorous by-products was minimized or completely eliminated.